DateTime Wrappers for PGObject

PGObject::Type::DateTime provides a basic wrapper around DateTime to allow
PGObject-framework types to automatically tie date/time related objects, but
date and timestamp formats are handled in the 'from_db' routines.

This specific module only supports the ISO YMD datestyle. The MDY or DMY
datestyles may be usable in future versions but datestyles other than ISO
raise ambiguity issues, sufficient that they cannot always even be used in
PostgreSQL as input.

This module also provides basic default handling. Times are assigned a date
of '0001-01-01' and dates are assigned a time of midnight. Whether this is
set is persisted, along with whether timezones are set, and these are
returned to a valid ISO YMD format on export, if a date component was
initially set.

This means you can use this for general math without worrying about many of
the other nicities. Parsing ISO YMD dates and standard times (24 hr format)
is supported via the from_db interface, which also provides a useful way of
handing dates in.